1. Field of the Invention
This invention relates to internal combustion engines and specifically to an improved gas turbine engine incorporating constant volume combustion for use in automotive and larger applications.
Gas turbine engines are used in many applications, but in the largest use of internal combustion engines--automobiles--they have not been commercially successful.
Presently, gas turbines use the excellent Brayton thermodynamic cycle (constant pressure combustion) and have performed well in large industrial and aircraft applications. However, there are limits to efficiency improvements and operation of this type of turbine which have prevented its use in some applications.
Another type of gas turbine incorporates a "constant volume combustion" cycle. The higher temperatures and pressures obtained from a specific air-fuel ratio in constant volume combustion enables great potential performance. This cycle can result in fuel savings and lower emissions than that obtained in present gas turbines, if the ensuing energy is utilized effectively.
A number of gas turbine designs have been patented in the past which utilize constant volume combustion including those referenced herein. The problems preventing their general use include high cost, waste of available energy present in combustion gases, idle and part-load performance, low specific power, inefficient process flow, minimal process improvements and cumbersome mechanical components. One of the most important problems--waste of available energy--is described as follows.
In gas turbines, creating a high velocity is a necessary step in transforming available energy of combustion into work. Hot combustion gases are accelerated through nozzles or stators by imposing a pressure differential thereon. The resulting high velocity gas will then "spin" turbine wheels. Impulse blading, reaction blading, or unorthodox blading installed on the turbine wheels convert the high velocity gases' energy to rotational power. However if the pressure difference across the turbine nozzle is too great, gases leaving the nozzle will experience a wild and turbulent expansion with acquisition of little or no increase in velocity above that obtained by a lower pressure. (The maximum useful pressure ratio between the nozzle inlet and outlet is called the critical pressure ratio, and results in a gas velocity equal to the speed of sound. The critical pressure ratio for turbine combustion gases of interest is approximately 1/0.54=1.85/1, but will be referred to as approximately 2 to 1 for simplification in the following discussions.) A large part of the available energy is lost by wasting high pressures which cannot perform useful work if the turbine nozzle pressure differentials are too large. At the other extreme, if the upstream pressure is too low, the resulting low nozzle velocity will be ineffective in producing work.
2. Description of the Related Art
The above energy loss due to turbulence by having only one set of nozzles would occur in the constant volume combustion turbine described by U.S. Pat. No. 4,693,075 to Sabatiuk, Sep. 15, 1987 unless the combustion pressures were kept very low. Low gas pressures in turn would require large machines for the production of relatively small amounts of output power. (This is called low specific power). If it were possible to maintain a high, constant, upstream pressure, the number of turbine nozzle stages would be increased so as to divide the high pressure into efficient smaller pressure steps for each nozzle set. (Each pressure step being equal or somewhat less than the critical pressure ratio.) Automotive size and intermediate size turbines require high revolutions per minute (RPM) to generate power. No matter how many chambers are firing per revolution of U.S. Pat. No. 4,693,075 to Sabatiuk, Sep. 15, 1987 or what angular relationship is provided between combustor intake and discharge, there is generally insufficient time for the complete combustion cycle postulated during one revolution of the rotor. For example if it operates at 40,000 RPM, one revolution is equal to 0.0015 seconds.
U.S. Pat. No. 5,237,811 to Stockwell, Aug. 24, 1993 schedules the opening and closing of constant volume combustion chamber exhaust connections to take place twice during each revolution of turbine (44D). Time between combustor discharge openings is approximately 0.0008 seconds if this turbine were operated at a speed of 40,000 RPM. It is submitted that a complete cycle--intake, combustion and discharge--cannot be completed during one-half revolution of a high speed device such as this. Complete combustion is limited by the velocity of the flame propagation. The routing of changing gas pressures through a set of fixed vanes operating in the position of a gas nozzle in this machine results in continued off-design performance. Gas velocity vectors controlled by the nozzle pressure ratio must match the speed and shape of the turbine wheel blades for efficient performance.
U.S. Pat. No. 4,570,438 to Lorenz, Feb. 18, 1986 addresses the constant volume combustion nozzle pressure problem by installing several nozzles and turbine wheels in series. If pressures upstream of the nozzles were constant this would be an effective solution for obtaining efficient nozzle operation. However combustion chamber pressures will decay as gases flow to nozzles from any constant volume combustor, which prevents a single series of fixed nozzles (and turbine wheels) from providing an efficient available energy conversion. Various arrangements of nozzles and turbine wheels are shown in FIGS. 2 through 6A, but all arrangements provide for flow of gases in a series disposition. U.S. Pat. No. 4,570,438 to Lorenz, Feb. 18, 1986 also preheats the air prior to entering the combustor. In contrast to a constant pressure process, air preheating is detrimental in a constant volume combustion cycle. Preheating will reduce the air density, decreasing the mass of air charge, in turn lowering the volumetric efficiency and specific power. The use of turbine blading as an exhaust fan is inefficient as the concave blade is turned in the wrong direction.
U.S. Pat. No. 3,791,139 to Simons, Feb. 12, 1974 incorporates a rotating combustor wheel with elongated, slender, constant volume combustion chambers. The patent describes this engine as "designed primarily for a metallic dust and gel fuel". The elongated, cylindrically shaped combustor rotor has nozzles and turbine blades built into each end of the rotor which are intended to propel it and provide output power through a central shaft. The machine does not include air compression or a means of cylinder purging, and does not address losses due to the declining pressure of constant volume combustion.
Additional References Cited.
U.S. Pat. No. 2,937,498 to Schmidt, May 24, 1960 preferred embodiments are turbo-jet applications. Two sets of combustors with end controls rotating at main shaft speed and operating in series produce substantially constant volume combustion. Combustors discharge into two sets of tubes supplying two coaxial gas streams to a single turbine blade ring which cannot efficiently handle two gas streams widely varying in pressure or velocity. The cumbersome tubing physically limits the number of annular gas streams (two) with originating pressure values (two) produced by the combustors, said two pressure steps cannot fully expand a high combustion pressure in relatively small steps except in inefficient, oscillating, series flow streams.
U.S. Pat. No. 3,417,564 to Call, Dec. 24, 1968 preferred embodiments are turbo-jet applications. Long narrow combustors, amenable to small engine cross-sections for low external aerodynamic drag, are used. A very high aspect ratio combustor configuration does not promote complete, efficient, combustion. This turbine requires two compressors, depends on a separate air pressure source to scavenge exhaust gas from combustor, and requires a dedicated exhaust system that is separate from the gas flow to the turbine wheels. Complex inlet and exhaust manifolds connected to the combustors incorporate scavenging means. A single annular gas flow stream is supplied to the turbine wheels, negating the possibility to discharge multiple gas streams which can fully and effectively utilize the variable pressures produced by constant volume combustion.
U.S. Pat. No. 3,611,720 to Fehleu, Dec. 12, 1971 describes a gas turbine utilizing a split combustion chamber which is half rotating and half fixed, having jet nozzles built into the rotating half. All fixed nozzles are subjected to the variable pressures which produce changing velocities and off-design performance. One revolution of the high speed main shaft provides limited time for a complete combustion cycle. A single annular gas flow stream is supplied to the turbine wheels, negating the possibility to discharge multiple gas streams which can fully and effectively utilize the variable pressures produced by constant volume combustion.
U.S. Pat. No. 4,241,576 to Gertz, Dec. 30, 1980 describes a turbo-jet engine incorporating constant volume combustion between the blades of a centrifugal fan which then discharges to a single annulus supplying the turbine wheels, negating the possibility to discharge multiple gas streams which can fully and effectively utilize the variable pressures produced by constant volume combustion. Four combustion cycles per revolution of the high speed main shaft provides insufficient time for complete combustion.
U.S. Pat. No. 3,811,275 to Mastrobuono, May 21, 1974, describes a complex gas turbine device in which jet nozzles are built into a combustor rotating at main shaft speed and producing two cycles per revolution. The combustor jets contributing to engine output are subjected to declining chamber pressures, creating changing velocities and fluctuating impulses. Different embodiments incorporate tortuous gas flow routes through various mechanical devices.
U.S. Pat. No. 4,620,414 to Christ, Nov. 4, 1986 incorporates a low pressure air supply limiting the combustion discharge gas pressure regardless of combustion temperature influence. The combustor, which is built in two pieces, contributes to sealing problems and the relatively small combustion chambers foster low specific power output. A single annular gas flow stream is supplied to the turbine wheel, negating the possibility to discharge multiple gas streams which can fully and effectively utilize the variable pressures produced by constant volume combustion.
U.S. Pat. No. 736,715 to Gervais, Aug. 18, 1903 utilizes a non-rotating combustor, poppet valves and a reciprocating valve actuating mechanism, limiting engine speed and gas production, and producing low specific power. A single annular gas flow stream is supplied to the single turbine wheel, negating the possibility to discharge multiple gas streams which can fully and effectively utilize the variable pressures produced by constant volume combustion.
U.S. Pat. No. 1,113,611 to Scheuer, Sep. 7, 1961 embodies a gas turbine utilizing an axial flow air compressor and a combustor rotating at high shaft speed and producing two combustion cycles per revolution with insufficient time to optimally execute intake, ignition, and expansion. A single annular gas flow stream is supplied to the turbine wheels, negating the possibility to discharge multiple gas streams which can fully and effectively utilize the variable pressures produced by constant volume combustion.
Prior art constant volume combustion turbines generally fall into the following categories:
relatively high combustor exhaust pressure diluting intake air and making intake difficult PA1 declining combustor pressures resulting in large energy losses in fixed turbine nozzles PA1 insufficient time for combustion due to high speed of combustion cycles PA1 preheating air to combustor lowering volumetric efficiency PA1 complicated or cumbersome hardware designs PA1 inefficient, unorthodox turbine blade designs PA1 a low rate of combustion gas production PA1 complicated air and gas flow routing PA1 special fuel requirements
A major shortcoming of turbine prior art is in not addressing the declining pressures inherent to the discharge of a constant volume combustion chamber, resulting in wasted energy. The problem of declining gas pressure resulting in inefficient nozzle and turbine operation is not readily apparent and may have been overlooked in past designs. In other cases, this problem may have been recognized, but ignored due to lack of an available solution. Gas turbine efficiency, performance, and cost factors must be outstanding in order to justify the huge cost required for testing, and adding to tooling and production methods now in place.